Patent GB-1294280-A (GB)
Title: Testing circuits
Inventor: HEILWEIL MELVIN FREDERICK
IPC: G01R31/3193
Filed: 1971-04-19
Abstract: 1294280 Testing logic circuits INTERNATIONAL BUSINESS MACHINES CORP 19 April 1971 [12 May 1970] 27187/71 Heading G1U In a logic circuit tester of the type in which the output of a circuit under test in response to an input test pattern is compared with the output from a reference circuit in response to the same test pattern, a fault signal may be produced erroneously because of the existance of "race" conditions within the circuit, i.e., conditions which result in one or more output bits having their values determined unpredictably by the relative speeds of response of various parts of the circuit, rather than uniquely by the input pattern, since in these conditions although the circuit under test may not be faulty its output may differ from that produced by the reference circuit. The production of these erroneous fault signals is avoided, in the invention, by identifying which output bits are determined by race conditions before the outputs from the circuit under test and the reference circuit are compared, and omitting these bits from the comparison. An example of a circuit which exhibits race conditions is shown in Fig.2. With an input pattern of (A, B, C) = (O, 0, 0) the output D is uniquely determined as 0. If now the B input bit changes to 1, then the output bit D may either change to 1, as shown in Fig.2A, if NOR gate 213 switches before NOR gate 214, or bit D may remain 0, as shown in Fig. 2B, if NOR gate 214 switches before NOR gate 213. An n-level logic circuit under test is simulated by a reference logic circuit having (n+1) levels, and each transition from a first level to a second level in any of the n-level bits applied to the n-level circuit under test is accompanied by two transitions in the corresponding input bit to the reference circuit, these being from the first level to the additional level, and from the additional level to the second level. Thus if the circuit of Fig.2, is simulated by a reference circuit Fig.3, in which the NOR gates are designed to receive, and to produce, trinary bits O, X, 1, represented, by negative, zero, and positive potentials, then when bit B of the circuit under test changes from 0 to 1, bit B of the reference circuit is made to change from O to X, and then, Fig.3A, from X to 1. The truth table for the 3-level NOR gates is as shown in Fig.13, and the resulting output bit D from the reference circuit after the transitions of bit B from O to X and from X to 1, can be seen to be X, the presence of which signifies that in the two-level circuit under test the output bit D is subjected to race conditions for the particular transition considered. In the preferred embodiment instead of using three amplitude levels for the three-level reference circuit, each of the three levels in binary coded by two bits, as in Fig.8, and each three-level NOR gate is simulated by two NAND gates. Thus, in Fig.7  (not shown), which simulates the circuit of Fig.3, NAND gates 701 and 702 simulate the three-level NOR gate 311 of Fig.3, and the two-bit binary coded output dd' corresponds to the output D of Fig.3. The transitions discussed above with respect to Fig.3, are summarised in Fig.9 for the circuit of Fig.7 (not shown). The overall system of the invention is shown in Fig.4 and is essentially a modification of the tester described in specification 23368/70, (Serial No 00000/00). The input bits applied to a circuit under test 20, such as the circuit of Fig.2, are sequentially changed, one-by-one, under the control of a random number generator 10 and a clock 50. Each single transition applied to the circuit under test is also applied to a unit "converter to 3-level logic" which, as a consequence, applies two transitions to the 3-level simulated reference circuit 409. Those output bits of the reference circuit which remain at the additional level, and which therefore indicate those output bits of the circuit under test that are subject to race conditions, are detected and used to inhibit subsequent comparisons involving such bits, and the remaining output bits from the simulated reference circuit are converted to 2-level logic, 405, and compared 40 with the outputs from the circuit under test.